Aromatic copolyestercarbonates are well known as copolymers derived from carbonate precursors, dihydric phenols and aromatic dicarboxylic acid or acid derivatives. Among the properties characterizing these polymers is a higher distortion temperature under load (DTUL) than for aromatic polycarbonate. When the simplest aromatic dicarboxylic acid or acid derivatives are employed, it has generally been found that a higher DTUL is achieved when more of the para-aromatic dicarboxylic acid, terephthalic acid, is employed as opposed to the meta-aromatic dicarboxylic acid, isophthalic acid. If a DTUL of about 163.degree. C. is desired, it can be achieved with an aromatic copolyestercarbonate derived from bisphenol A and having approximately 72 weight percent ester of which 85% of the ester is terephthalate and 15% is isophthalate. This particular polymer has reasonable impact values in a Notched Izod system. Both the 3.2 millimeter and 6.4 millimeter thickness test pieces break in a brittle failure mode at approximately 32.6 kgf cm/cm of notch. This copolyestercarbonate retains a substantial number of bisphenol A polycarbonate properties to a great degree. However, this particular aromatic copolyestercarbonate composition displays unfavorable stress cracking of molded parts under many circumstances. This stress cracking can be caused by, inter alia, autoclaving, temperature cycling, and molding under ordinary processing conditions. Under these testing conditions bisphenol A polycarbonate shows excellent resistance to stress cracking.
It was therefore desirable to prepare an aromatic copolyestercarbonate which maintained as many of the positive properties of the high terephthalic acid containing copolyestercarbonate, for example the high DTUL, while substantially improving the stress cracking properties.
Initially the copolyestercarbonate which substantially reduced the stress cracking problems was an 80 weight percent ester copolyestercarbonate derived from approximately 100% isophthalate units. However, it was found that the impact resistance of this copolyestercarbonate at the 3.2 millimeter thickness could not be consistently maintained from preparation to preparation and appeared to be somewhat dependent upon molecular weight. Additionally, another problem occurred during the scaleup of the reaction to produce 9-11 Kgs of copolyestercarbonate in a single batch. Although the reaction was carried out in methylene chloride in a typical interfacial polymerization similar to that used in preparing aromatic polycarbonate and in accordance with the general disclosure of Quinn U.S. Pat. No. 4,238,596, the product copolyestercarbonate is difficult to resolubilize in methylene chloride. Nesistance, at least in the 3.2 mm section.